Re: but does the *earth* push up on the *wing*?

[Timothy Folkerts <tfolkert@TIGER.FHSU.EDU>]

> As I have said in later message,take the case of a plane going up at a
> high speed.  At the top the pilot pushes the stick forward hard and
> experiences -6g.  Clearly the wings are pulling the plane down.  They
> cannot be pushing on the earth.  The physics of flight does not change at
> the top and the bottom of a loop.

I'm not quite sure what your point is.  Let me say a couple things.

> The physics of flight does not change at the top and the bottom of a loop.
1.  I don't know who's model of lift is better.  I'm sure either one has
good things to say about exactly how +-6 g are actually produced.  But I
think that is a separate issue from the third law question.

2.  Now to the third law.  I guess my point here is how immediate do the
cause and effect have to be in order to be considered an action-reaction
pair?  If I pull on your hands, is that an action-reaction pair?  What if
we are wearing gloves?  What if we pull on opposite ends of a rod?  What if
we pull on opposite ends of a long, stretchy spring?  It's mostly a matter
of personal preferrence as tio where you draw the line, but in every case,
the force at one end is transmitted to the other.

Now, the airplane is admittly at the "stretchy spring" end of the spectrum.
But I bet for a plane flying near the ground at constant speed, you could
measure the the pressure distribution and if you integrate
delta(Pressure)dA at any given instant, and you would get mg.  In
principle, if the plane was flying higher, the pressure changes would be
less and the area it is spread over larger, but it would still be mg.  In
principle, plane flying loop-the-loops at +- 6g would cause pressure
changes which would vary at the same frequency as the loop-the-loops.
There may be a considerable delay, and there may be considerable damping,
but the average of the integral delta(P)dA would still be mg.

> Clearly the wings are pulling the plane down.  They
> cannot be pushing on the earth.
3.  Part 2 doesn't say that pushing on the earth is a necessary condition
for flight, anymore than it says that a rocket needs to push on the earth
to fly.  But in either case, as long as particles shot downward are
"caught" by the earth, the ultimate fact is that they DO apply a time
averaged force to the earth exactly equal to the time averaged force
applied to the plane and/or rocket.


Tim